[LinuxBIOS] SMP hardware question

[Adam Talbot]

Currently I have a dual proc Tyan MPX system running two athlon XP's 
that I have converted to MP by closing a bridge on the top of the CPU's.

So that little trick changed the name of the CPU, but what makes an XP 
CPU not capable of running in an SMP setup?  Is this a factor of the 
BIOS or is there something more?
-Adam Talbot